This invention relates to the measurement of rotary motion and, more particularly, to a device for generating an electric alternating signal representative of rotary motion. This device includes a signal generator ring which rotates with the part undergoing the rotary motion and a stationary inductive transducer which is arranged within the surface defined by the signal generator ring and which has a double-sided measuring system with a measuring coil, a coil core and pole shoes.
One device used to measure rotary motion is described in German Published Patent Application (Offenlegungsschrift DE) No. 39 27 007A1.
In that reference, a toothed disk with inwardly directed teeth serves as the signal generator ring. Inside the disk, there is a diametrically aligned measuring coil. The coil encloses a magnetic flux guide piece the length of which approximately corresponds to the inside diameter of the disk, reduced by the necessary distance to allow an air gap between the ends of the magnetic flux guide piece and the inwardly directed teeth of the signal generator ring. The magnetic flux guide piece protrudes from both sides of the coil. Permanent magnets, oriented such that they are magnetized perpendicularly to the axis of the magnetic flux guide piece, are placed on both sides of this magnetic flux guide piece at the very ends thereof. The magnets are thus also disposed close to the inwardly directed teeth. If there are two permanent magnets at a time on the sides of the ends of the magnetic flux guide piece, these magnets are arranged so that both of them have the same pole on the opposite sides of the magnetic flux guide piece and, consequently, point outwards with the same pole. As the indicator ring rotates, when both magnets of one coil side are simultaneously directly opposite teeth extending inwardly from the ring, a magnetic flux forms from the outside of the magnets through the toothed disk to the magnetic flux guide piece, and then to the inside of the magnets. When the tooth gaps are opposite the magnets, the main flux passes from the outside of the magnets through the air in the space created by the tooth gap to the magnetic flux guide piece and to the insides of the magnets. In other words, the magnetic flux passes to the magnetic flux guide piece and to the magnet pole seated on the guide piece either via a tooth if it is a tooth that is opposite the individual magnet, or via air if it is a tooth gap that is opposite the magnet. Thus, on the two coil sides the magnetic flux is alternating through the magnetic flux guide piece in dependence on whether tooth gaps or teeth are opposite the magnets. Thus, the changes in the magnetic flux that are caused by the individual permanent magnets in cooperation with the individual teeth are, in a sense, superimposed upon one another. The achievable value of the measuring signal depends on the number of the permanent magnets used.
Another device used to measure rotary motion is described in German Published Patent Application (Offenlegungsschrift DE) No. DE 35 42 962 A1. This reference describes a magnet core inserted into the measuring coil of an inductive transducer. The magnet core is magnetized perpendicularly to the coil axis and has on its pole surfaces, parallel to the axis of the measuring coil, plate-shaped pole shoes. The pole shoes protrude from the side of the coil facing the outwardly directed teeth of a signal generator ring. The measuring signal in such a measuring arrangement, where the motion to be measured is only on one side of the measuring coil, essentially depends on the air gap between the pole shoes and the teeth and, thus, on assembly tolerances.